In many situations it is important to be able to detect the presence of inhomogeneities, such as bubbles, in a liquid flow. This is the case in, for example, the medical field in parenteral administration of treatment solutions where the patient must be protected from the infusion of air bubbles. In industry, for example, the presence of bubbles in a cooling system indicates insufficient cooling capacity. In the analytical field, particularly the detection of air bubbles in passages for liquid chromatography may be mentioned.
A common method for detecting bubbles of gas in a liquid flow is based on the use of ultrasonics and relies on the fact that a gas has a considerably higher acoustic impedance than that of a liquid or a solid material. Thus, if ultrasound is emitted from a sender on one side of a liquid conduit to a receiver on the other side of the conduit, the presence of air bubbles, for example, in the liquid may be detected as a distinct reduction of the received sound energy compared to when there is only liquid in the conduit.
Ultrasound may be generated in a piezoelectric transducer, so-called piezotransducer, in which a crystal is oscillated when actuated by an electric voltage. Conversely, the crystal produces an electric voltage when ultrasound hits the crystal. An ultrasonic transducer may consequently both emit and receive sound.
There is previously known for liquid chromatography purposes an apparatus for bubble detection, which apparatus can be connected to a chromatography system and comprises a plastic block having a liquid through-passage, and a sender and a receiver mounted to either side of the liquid passage. A disadvantage of this apparatus is that the difference between the signals obtained for liquid and air, respectively, is not very great since the emitted ultrasound can propagate beside the liquid passage and still hit the receiver. Likewise, ultrasound that has passed through the liquid passage can then propagate beside the receiver. The apparatus therefore requires complicated electronics for processing the signal from the ultrasound transducers as well as a complicated trimming procedure when installing the device.
U.S. Pat. No. 4,418,565 describes an ultrasonic bubble detection apparatus in the form of a plastic block having inserted therein a pair of opposed ultrasonic transducers (transmitter and receiver) and a channel defined between the transducers, into which channel the liquid conduit where bubbles are to be detected is applied, typically, a tube from a bag or bottle for parenteral administration of solution. Between each transducer and the passage there is a recess filled with an elastomeric ultrasound-transmitting material. To prevent propagation of ultrasound from the sender to the receiver by a route other than through the elastomeric material, and thereby through the liquid conduit, an air-containing slot is positioned in the bottom of the channel. This slot, which is too narrow to be capable of receiving the liquid conduit, extends to a depth at least below the lower edges of the ultrasonic transducers.
While this construction for guiding the ultrasound by shielding at least partially overcomes the above mentioned detection problems of the first described bubble detecting apparatus, the construction has other disadvantages. First of all, the apparatus itself has a complicated construction and contains inter alia screws, spacer elements, and pressure plates. Further, the sensing of a tube is unpractical and requires that the tube be mounted with good acoustic coupling. This may in fact be accomplished if the tube is flexible, but such a tube does not withstand the relatively high fluid pressures that often prevail in, for example, liquid chromatography. The achievement of sufficient acoustic coupling with a rigid tube, on the other hand, requires the use of mounting paste which is very impractical. Due to the slot defined below the liquid conduit channel, the bubble detection apparatus also obtains bad strength properties, especially with regard to twisting or torsional strength.